This invention relates generally to controlling redundant hardware in communication systems and more particularly to using wireless data transmissions from a transmitter system to control switching between redundant modulator units.
Various issued patents and pending applications including U.S. Pat. Nos. 4,713,808 and 4,897,835 (both by Gaskill), U.S. Pat. No. 5,187,470 (King) and pending application Ser. No. 08/046,112, filed Apr. 09, 1993 show systems where digital data is transmitted on an FM subcarrier and modulated on an FM baseband. The data is transmitted to a pager receiver that filters the subcarrier from the rest of the FM baseband and then decodes the data on the subcarrier into digitally encoded messages.
To increase reliability, systems as discussed above often use redundant transmission equipment. For example, FIG. 1 is a prior art schematic diagram of a wireless paging system such as shown in U.S. Pat. No. 4,713,808 to Gaskill et al. that uses redundant modulator units.
Modulator units 18 and 24 each receive the same transmission data from a local clearinghouse facility 11. The modulator units each separately encode the transmission data into time division multiplexed data packets that are carried on a subcarrier signal. Each modulator unit 18 and 24 includes a relay (not shown) that selectively couples the modulator unit output to a frequency modulation (FM) exciter 28. The FM exciter 28 combines the subcarrier signal with an FM audio signal 35 onto an FM baseband.
The FM baseband output from FM exciter 28 is amplified by a power amplifier (PA) 30 and then broadcast from antenna 32 to a remote receiver. For example, the digital data output from system 12 is transmitted to wireless pager receivers.
Modulator unit 18 and 24 are connected to a receiver 26 that monitors the signal transmitted from antenna 32. An inter-unit communication bus 20 is used for relaying general control information from a central controller (not shown) to both modulator unit 18 and 24. Dedicated redundancy status lines 22 are coupled between modulator unit 18 and modulator unit 24 to communicate modulator unit operational status.
When modulator unit 18 is actively coupled to FM exciter 28, receiver 26 monitors the condition of the output from modulator unit 18 transmitted from antenna 32. If data is not being accurately transmitted, for example, modulator unit 18 has failed, modulator unit 18 sends a signal over lines 22 instructing modulator unit 24 to connect to FM exciter 28. In turn, modulator 18 disconnects from the FM exciter.
Since modulator unit 24 is generating the same data on the same subcarrier signal output by modulator unit 18, data communication to remote receivers is maintained by replacing the failed modulator unit 18 with backup modulator unit 24.
The problem with the redundant system shown in FIG. 1 is that dedicated redundancy control lines 22 are required to control operating status between modulator unit 18 and modulator unit 24. Control lines 22 are used to provide the modulator unit power state, relay state, relay control, and handshaking data between the two modulator units 18 and 24. Using dedicated redundancy status lines 22 increase the number of pins and overall hardware complexity of each modulator unit.
General purpose communication networks such as a local area network (LAN) are effective for carrying information between different transmission systems. However, LAN's frequently fail or delay message transmissions during heavy data traffic. As a result, a general communication network such as a LAN does not have sufficient reliability or operating performance for transmitting redundancy status information.
Accordingly, a need remains for communicating redundancy status information to redundant modulator units without increasing hardware complexity or reducing reliability in the transmitter hardware.